Reinforced thermosetting polyester-based molding compositions in the form of sheet molding compound (SMC) and bulk molding compound (BMC) have been known for many years. These materials are based on unsaturated polyester resins produced from a reaction between a polyol having at least 2 hydroxyl groups, and a mixture of saturated and unsaturated dicarboxylic acids (or their anhydrides). The initially formed unsaturated polyester resin is blended with one or more monomers capable of crosslinking with the unsaturated in the polyester, a peroxide catalyst, and a reinforcing material such as fiberglass, then heated to decompose the peroxide and cause the crosslinking reaction between the monomer and the unsaturation in the polyester molecule to occur. The resulting product is a composite of the reinforcing material and the crosslinked polyester.
Thermoplastic materials are often added to the molding compositions in order to reduce shrinkage of the part during cure, or in some cases a small amount of expansion, thereby providing molded parts which more accurately reflect the molds in which they were made, and which have relatively smooth surfaces. These thermoplastics are referred to in the art as "low profile additives".
A number of thermoplastics have been found to give varying levels of shrinkage control. Examples are poly(vinyl acetates); polymethylmethacrylates and copolymers with other acrylates; copolymers of vinyl chloride and vinyl acetate; styrene-butadiene copolymers and other elastomers; polystyrene and certain copolymers of certain monomers; polycaprolactones; cellulose acetate butyrate; saturated polyesters and various blends of saturated polyesters with poly(vinyl chloride); and polyurethanes.
In molding compositions containing polyurethane low profile additives, the polyurethane is typically prepared by first reacting an organic polyisocyanate with a polyester or polyether polyol to form a hydroxyl-terminated prepolymer and then reacting this with a dicarboxylic acid anhydride to form an esterified polyurethane.
In addition to the thermoplastics, the thermosetting polyester resin compositions often also contain a variety of additives, including pigments. However, the effects of the polyurethane additives on the pigmentability of polyester-based thermosetting resin compositions can result in molded articles which have a hazy, blotchy or agglomerated appearance (referred to in the art as mottling).
Accordingly, improved polyurethane low profile additives are desired which can provide molded articles with improved pigmentability characteristics in addition to shrinkage control.